1. Field of the Invention
The present invention is directed to RADAR applications, and, more particularly, to detection and mitigation of cross-eye jamming of RADAR signals.
2. Description of the Related Art
Radio detection and ranging (“RADAR”) is a well-known remote sensing technology. RADAR was originally developed for military applications in which enemy units could be detected at ranges longer than could be achieved through direct, line of sight visual observation. Although RADAR still finds many military uses, it has made its way into a number of civilian contexts as well.
In very general terms, a typical, transmitted RADAR signal is what is known as a “sum signal”. The sum signal is reflected from, for example, a target, and received by the RADAR. The RADAR receiver generates several signals from the return, one of which is known as a “sum signal,” sometimes represented by the symbol “Σ” and another known as a “delta signal”, “Δ”. The sum signal and delta signal can then be processed to determine the angles in azimuth and elevation from the RADAR to the target. In a classic case, a missile targeting an aircraft using RADAR, for example, can use these angles to guide the missile to the aircraft.
Sometimes, and particularly in military applications, it is desirable to interfere with the RADAR's use. In the classic case mentioned above, the aircraft would typically want to avoid being hit by the missile. The aircraft might employ a number of countermeasures. Some of these countermeasures may interfere with the operation of the RADAR.
One type of countermeasure is to “jam” the RADAR signal, and one well known kind of jamming is known as “cross-eye jamming.” Cross-eye jamming affects monopulse antenna systems by causing errors in angle indication that are large enough to cause loss of track. In cross-eye jamming, the target intercepts the transmitted RADAR signal, manipulates it, and returns it to its source. The target manipulates the RADAR signal so that, when received and processed, the resultant sum signal looks like the delta signal should and the delta signal looks like the sum signal should. This, in turn, yields incorrect angle determinations.
More particularly, consider the missile/aircraft scenario again. When the missile is tracking the aircraft on boresight, the sum signal will show a maximum at the boresight and the delta signal will be zero. Cross-eye jamming, however, will cause the resultant sum signal to be zero on boresight and the delta signal to have a maximum on boresight. Thus, the missile will think the aircraft is no longer on boresight, erroneously turn off its flight path, and lose the aircraft.
A fuller explanation of cross-eye jamming is presented in U.S. Pat. No. 6,885,333, issued Apr. 26, 2005, to ITT Manufacturing Enterprises, Inc. as assignee of the inventors Mitchell J. Sparrow and Joseph Cikalo. This patent is hereby incorporated by reference for teaching the basic principles of cross-eye jamming as if set forth verbatim herein.
While cross-eye jamming brings a desirable result from the perspective of the target, the result is highly undesirable to the target's opponent. The target's opponent would therefore like to have a way to at least detect and, hopefully, to mitigate the effects of cross-eye jamming.